I've noticed that common flies immediately collapse in cold climates. At the cold aisle of a grocery store, you can see countless dead flies on the bottom of the shelf. Tonight I opened my freezer and a fly flew inside, and less than five seconds later he fell to the bottom and lay motionless. I grabbed him by the wing and blew hot air on him, and he began to try to fly.

Why do flies stop functioning so quickly in the cold?

I've also heard of bees doing this. My friend told me that if you catch a bee and put it in the freezer, you can tie a string around it and let it "thaw," then carry it on a leash.

During flight, the insect thorax has a temperature a lot higher than that of the other body parts. This high temperature is required to be maintained for the purpose of flight. If you suddenly decrease the temperature, I suspect its wings should start beating slower and slower and eventually its wings shouldn't be able to sustain flight.

Flies, being cold-blooded animals, become slower the colder their environment is, and naturally flying requires some speed. So when the fly becomes so cold it cannot flap its wings fast enough it falls down. If it gets colder still, its other movements become slower and slower until they cease completely.

Cold-blooded animals do not usually die very soon even if they end up in a cold place and cannot move anymore. Their vital functions just slow down very much, but they only need some warmth to get going faster again.

The fly has such a small mass that it gets cold very soon in cold places, thus they collapse quickly.

It is also worth mentioning that many cold-blooded animals are well-adapted to living in cold environments: fish and curstaceans live in icy waters and some northern insects can walk on snow and ice without problems.

I suspect the flies you have observed are found in some warm or temperate place, not in, say, Greenland? (: